1. Field of the Invention
The present invention relates to an optical transmission system, an optical transmission module, an optical modulator, and method of driving optical modulator.
2. Description of the Related Art
The recent developments in optoelectronic technology, including dispersion-shifted fibers, high-speed optical modulators, and erbium-doped fiber amplifiers, have enabled long-haul fiber transmission of ultrahigh-speed signals in the 1.55 .mu.m-wavelength band. In the areas where non-dispersion-shifted fibers have already been laid, however, even when an external modulator with a small wavelength chirp (a wavelength shift due to modulation) is used, a high-speed signal whose bit rate is 10 Gb/s or more cannot be transmitted over a long distance, making it difficult to increase the transmission capacity in a cost-effective manner as the amount of information will increase in the future.
According to P. S. Henry, IEEE J. Quantum Electronics, vol. QE-21, December, 1985, pp. 1862-1879, the limit distance L of fiber transmission in the case of chirpless modulation is expressed by: EQU L.ltoreq.c/(2D.lambda..sup.2 B.sup.2) (1)
where c is the speed of light, D is the dispersion of the fiber, .lambda. is the wavelength, and B is the bit rate. This limit causes from modulation sidebands.
For example, if signal transmission at a bit rate of 10 Gb/s is to be performed by the use of a non-dispersion-shifted fiber having a wavelength dispersion of about 15 ps.multidot.km.sup.-1 .multidot.nm.sup.-1 at a wavelength of 1.55 .mu.m, where erbium-doped amplifiers can be used, a signal can be transmitted only as far as about 40 km unless suitable dispersion compensation is effected.
Even when an external modulator is used, actual optical modulation involves wavelength chirping. Chirp parameter .alpha. is used as a quantity indicating the ratio of phase modulation to intensity modulation. When an electroabsorption semiconductor optical modulator is used, .alpha. changes greatly, depending on the voltage. From the optical fiber transmission characteristic, however, art equivalent chirp parameter .alpha. can be defined. If .alpha. is in range of 0 to -1, suitable pulse compression will take place, making the transmission limit distance longer a little (see A. H. Gnauch et al., IEEE Photonics Technology Letters, vol. 3, October, 1991, pp. 916-918). As compared with the case of .alpha.=0, however, the transmission limit distance in signal transmission at a bit rate of 10 Gb/s is improved to the extent that it gains only a stretch of about 10 km.
Furthermore, in the case of electroabsorption semiconductor modulators, the equivalent chirp parameter .alpha. often takes the value ranging from +0.5 to +2, with the result that the transmission limit distance is shorter than when .alpha.=0. Recently, it has been reported that with an electroabsorption semiconductor optical modulator, the expression .alpha.&lt;0 can be fulfilled by bringing the working wavelength and the peak wavelength of exciton absorption closer to each other than in the prior art (see J. A. J. Fells et at., Electron Letters, vol. 30, July, 1994, pp. 1168-1169). Since this method involves a decrease in the extinction ratio and an increase in the insertion loss, the transmission limit distance does not get as longer as expected.
To break down the limits of the transmission limit distance due to such waveform dispersion, various dispersion compensation techniques have been proposed. However, most of them are complicated and expensive and the optical power penalty is large.
The simplest method is a dispersion compensation by connecting a fiber with an inverse large wavelength dispersion. A dispersion compensation fiber whose wavelength dispersion is about -70 ps.multidot.km.sup.-1 .multidot.nm.sup.-1 has been developed. With the method, however, the length of the optical fiber must be increased about 25% more than the actual transmission distance. On the other hand, when the gain of the optical fiber amplifier is increased to compensate for an increase in the aforementioned loss, the power penalty gets larger as a result of an increase in noise.
As described above, in the prior art, it is difficult to transmit a high-speed binary digital NRZ signal whose bit rate is 10 Gb/s or more over a long distance by the use of an optical fiber with large wavelength dispersion. Although dispersion compensation makes the transmission limit distance longer, this results in increases in the cost and the power penalty.